In 1964, the U.S. Bureau of Reclamation documented the first known use of controlled low-strength material (CLSM). Plastic soil-cement, as the Bureau called it, was used as pipe bedding on over 320 miles of the Canadian River Aqueduct Project in northwestern Texas. Since 1964, CLSM has become a popular material for projects such as structural fill, foundation support, pavement base, and conduit bedding.

CLSM is a self-compacted, cementitious material used primarily as a backfill in lieu of compacted backfill. Several terms are currently used to describe this material, including flowable fill, controlled density fill, flowable mortar, plastic soil-cement, soil-cement slurry, K-Krete, and other names. CLSM is defined as a material that results in a compressive strength of 1,200 pounds per square inch (psi) or less. Most current CLSM applications require unconfined compressive strengths of 200 psi or less. This lower strength requirement is necessary to allow for future excavation of CLSM.

The term CLSM can be used to describe a family of mixtures for a variety of applications. For example, the upper limit of 1,200 psi allows use of this material for applications where future excavation is unlikely, such as structural fill under buildings. Low density CLSM describes a material with distinctive properties and mixing procedures. Future CLSM mixtures may be developed as anticorrosion fills, thermal fills, and durable pavement bases.

CLSM is composed of water, portland cement, aggregate, and fly ash. It is a fluid material with typical slumps of 10 inches or more. It has the consistency of a milk shake.

Fast Discharge

Like most concrete, CLSM may be mixed in central-mix concrete plants, ready-mixed concrete trucks or pugmills. Once CLSM is transported to the jobsite, the mixture may be placed using chutes, conveyors, buckets, or pumps depending upon the application and its accessibility. A truck often can be discharged in less than five minutes. A constant supply of CLSM will keep the material flowing and will make it flow horizontal distances of 300 feet or more. Although CLSM may be placed continuously in most applications, care must be taken when backfilling around pipes. For pipe bedding and backfilling, CLSM is placed in lifts to prevent the pipes from floating. Internal vibration or compaction is not needed to consolidate CLSM mixtures. Its fluidity is sufficient to consolidate under its own weight.

The fluidity/flowability and self-compacting properties of CLSM mixtures make CLSM an economical alternative to compacted granular material due to savings of labor and time during placing. CLSM is also an all-weather construction material—it will displace any standing water left in a trench—making it a ideal material for many projects.

The primary application of CLSM is as structural fill or backfill in place of compacted soil. The flowable characteristics of CLSM mean that it can readily be placed into a trench and into tight or restricted-access areas where placing and compacting fill is difficult. CLSM also makes an excellent bedding material for pipe, electrical, telephone, and other types of conduits because the mixture easily fills voids beneath the conduit and provides uniform support.

CLSM will not settle or rut under loads, making the material an ideal pavement base. Additionally, CLSM can be placed quickly and support traffic load within hours of placement-minimizing repair time and allowing a rapid return to traffic. CLSM may be equal to or less than the cost of using standard compacted backfill.

Since 1979, the Iowa Department of Transportation has used CLSM to structurally modify more than 40 substandard bridges by converting them into culverts. CLSM is also used to fill large voids such as old tunnels and sewers. In a Milwaukee project, 830 cubic yards of CLSM were used to fill an abandoned tunnel.

A ready-mixed concrete producer can aid in developing a mix design for CLSM. However, when ordering CLSM, consider the following:

Strength: Applications that require removal of CLSM at a later date usually limit the maximum compressive strength to less than 200 psi.

Setting and Early Strength: Hardening time can be as short as one hour, but can take up to 8 hours depending on mix design and trench conditions.

Density in Place: Density of normal CLSM in place typically ranges from 90 to 125 pounds per cubic foot.

Flowability: Flowability can be enhanced through the use of fly ash or air entrainment.

Durability: CLSM materials are not designed to resist freezing and thawing, abrasive or erosive actions, or aggressive chemicals.